It could be fixed by escaping the namespace string before doing the split().sort().join()

// chars that should be escaped in RegExp constructor
var rEscapeRegExpChars = /[\\\\.+*?\\^$\\[\\](){}\\/'#]/g;

function escapeRegExp(str) {
    return str.replace(rEscapeRegExpChars,'\\\\$&');
}

// than later on the place where namespaces RegExp is created
namespaces = namespaces ? new RegExp("(^|\\\\.)" + escapeRegExp(namespaces).split("\\\\.").sort().join("\\\\.(?:.*\\\\.)?") + "(\\\\.|$)") : null;
//

I was trying to create a namespace to handle keys combinations like "keypress.ctrl+s" but the "+" breaks unbind() and off() since namespaces.test( handleObj.namespace ) will return false.

My solution for now was simply to avoid using chars that have special meaning inside RegExp (using "keypress.ctrl_s" instead).

Couldn't find any information about it in the documentation and it took a while to realize the the event wasn't being removed and that the problem was caused by the plus sign.